[v3] improve exception text when threads not enabled

[Joe Buck]

On Sun, Aug 12, 2012 at 08:02:30PM +0100, Jonathan Wakely wrote:
> This improves the fairly uninformative "Operation not supported"
> message given when std::thread is used without linking to libpthread.
> 
> Now you get:
> 
> terminate called after throwing an instance of 'std::system_error'
>   what():  Enable multithreading to use std::thread: Operation not permitted
> Aborted

The new message still seems deficient.  The issue is that the executable
does not contain any thread support; "not permitted" usually suggests a
permission violation (like trying to write a read-only file).  Perhaps "no
thread support found" should be used instead of "Operation not permitted".